1. Field of the Invention
This invention relates to a frequency division multiplex transmission signal receiving apparatus in a communication field or ground wave digital TV broadcast field and more particularly to a frequency division multiplex transmission signal receiving apparatus which is improved to enhance the receiving performance by reflecting a variation amount of a reception signal varying with time or frequency according to the transmission situation such as multipath or fading on an error correction process.
2. Description of the Related Art
In recent years, the technology in the ground wave digital transmission field dealing with audio and video signals is positively studied and developed. Particularly, in Europe and Japan, an OFDM (Orthogonal Frequency Division Multiplex) system which is one type of an FDM system attracts much attention as an optimum digital transmission (modulation) system. In the OFDM system, modulation and demodulation are performed while data items are assigned to a plurality of carriers which are perpendicular to one another, an inverse FFT (Fast Fourier Transfer) process is performed on the transmission side and an FFT process is performed on the reception side.
Each carrier can be modulated by a desired modulation system and a QAM (Quadrature Amplitude Modulation) system using synchronous detection and a transmission system using delayed detection can be used. In the synchronous detection, a pilot symbol whose property is known is periodically inserted into a transmission signal, a difference between the received pilot symbol and the pilot symbol before transmission is derived on the reception side and equalization of the amplitude and phase of the reception signal is effected. In the delayed detection, a differential coding is effected between reception symbols and the reception signal is demodulated without reproducing the carrier.
In digital transmission, it becomes essential to make error correction of a demodulated signal from the viewpoint of degradation of a signal in the transmission path and enhancement of the transmission characteristic.
In the conventional reception apparatus, error correction is made by using a Viterbi demodulation process optimally set for Gaussian random noise, but in some cases, it is impossible to fully exhibit the performance since the noise distribution is different from the Gaussian distribution in the case of fading transmission path in which a time variation amount of the demodulation signal is significantly large or because of the presence of reflection waves called multipath.
As described above, in the conventional reception apparatus, there occurs a problem that error correction cannot be sufficiently made in the case of a fading transmission path or the presence of reflection waves.